'Donatello, concentrate.'

The words are whiplash-sharp, and Splinter sees Donatello grit his teeth, the flicker of his eyelids that contains the ghost of an eyeroll – rebellion, frustration, anger.

Leonardo looks on, frowning, seeing what Splinter sees, yet not all of it. The boy kneeling, attempting to balance on his littlest finger the hilt of a katana; Raphael kneeling in a mirror image not one foot from his face, his katana steady where Donatello's wavers. Leonardo observes the distraction, the lack of form, but not the reluctance within Donatello, his lack of will to master that always keeps him a hair's breath, half a step behind his brothers.

It frustrates his teacher immensely.

Raphael's eyes crinkle at the edges, displaying a look calculated to be invisible to Splinter, but which he recognises nonetheless as a taunt. Donatello's shoulders become more rigid, he mouths something barely perceptible at Raphael – fuck. off. – and the blade stabilises.

'Up,' Splinter says, and the boys rise. 'Circle,' he says, and they circle each other in the dojo, feet moving fluidly, the swords winking in the low light. Then 'Arm,' and they each flip their weapon up and fall on each other with a clash of metal. Donatello falls back into a characteristically defensive while Raphael beats his way toward him with heavy, powerful strokes.

Donatello is forced to the ground, and besides him Splinter hears Michaelangelo's intake of breath as the sword swings down, and Donatello rolls.

It is not a lack of ability, but of interest.

It angers Splinter even as he knows it is an irrational response – Donatello's passions are multiple, the breadth and depth of them staggering, but ninjitsu has never been chief amongst them. He obeys his Sensei in matters of training, he does what is asked of him and much more where it affects his brothers, but he will never give his heart and soul to the discipline he has been trained in since he learned to walk.

Splinter feels the twinge of bitterness, and shakes it away – children are seldom grateful to their parents. And as his sons lunge and circle, then lunge again for each other, he realises he has allowed this fight to go on too long.

Both are sweating, both disarmed, and both angry. The air tastes of strained finality, and Splinter knows it is just a matter of time before Raphael gets close enough to pin Donatello beneath his superior bulk, as has happened countless times before. He can feel anxiety emanating from Michaelangelo at his shoulder – this puts them all in a bad mood – and raises a hand to forestall the inevitable.

'Enough.' They freeze, looking to him anxiously. He gestures them to the sidelines. 'Michaelangelo, Leonardo, to the mat.'

Raphael and Donatello face each other and bow – Raphael lower, a true bow, Donatello's a short, sharp jerk of the head.

They believe he cannot hear the words muttered under their breath as they trudge to the sidelines, but his ears are much more sensitive than even they know.

'Ass.'

'Jerk.'

Splinter sighs, and motions his other two sons to the centre of the ring, Michaelangelo swinging his arms, warming his muscles, Leonardo already narrow-eyed and focused.

He wishes for all his sons that this life were otherwise, but that one in particular wishes the same, and so deeply that he gives voice to it in a hundred silent ways, drives fear like cold blades through his belly. In the face of Donatello's boundless desires, what can a father do but teach him all he knows?

In the centre of the room, Leonardo body-slams Michaelangelo, who tumbles over his shoulder in a blur and comes up winded, but not vanquished. Raphael leans against the wall, arms folded, while Donatello sits cross-legged, elbows on his knees, his face turned obediently toward the fight, his eyes absent.

From the age of three, Splinter took his sons walking every week in the sewers outside the lair. This was for the combined reasons of exercise, increasing their familiarity with their immediate surroundings lest they become lost or separated, and a vague feeling that children should have, if not fresh air, then a change of air every so often.

They walked for an hour at a time, two children hand in hand directly ahead of, and behind him, each no further than an arm's length away. The sewers were dark, and gloomy, but for the children it was a carnival, and the walls rang with piping voices as they pointed out interesting objects – a broken shopping trolley, an iguana scurrying behind the pipes, a place where rushing waters formed an underground waterfall. And windows to the world above, circles of light cut from darkness like out of a dark cloth.

They were fascinated, though Michaelangelo clung to his hand as he watched clouds scudding across the sky, gulls dipping and diving on the wind, or people streaming along the street, holding coffee and umbrellas, faces cast down, away from the rain.

Splinter remembers endless queries, endless 'What's that?' and 'Why?'s. He remembers the four of them, so small and yet so curious, so brave.

One day they passed through a tunnel studded at 30-foot intervals with grates that showed Brooklyn in all its suburban glory – brownstone buildings, trees in leaf, a flutter of pigeons cooing and fighting in the gutter. He instructed the children to be silent as they passed so close to the surface, but he felt little need for concern – there was more than enough to look at to keep quiet.

Under the car exhausts and clatter of bakery vans, music was playing, childish voices uplifted in song – wheels on the bus go round and – and just as he smiled to hear the familiar song, Leonardo's squawk of distress froze his blood.

'Sensei – Donnie! DONNIE.'

Behind him, Donatello had climbed up the wall until he was level with the grate, hanging on to the bars and humming, 'wheels on bus, wheels,' his gaze riveted on the class of pre-schoolers sitting in the fall sunshine, following their teacher in the motions of the song. Time slowed, stretched to a white line of horror as the little turtle reached out to wave, opened his mouth.

'Donatello, no!'

Splinter scooped him up in time to pull him away – the Donatello up at him, and pointed, beaming. 'S'ool! S'ool Sensei!'

Where had the child picked this up? A book?

'Donatello, be silent.' He pitched his voice low and stern, but Donatello began to thrash in his grip, arms outstretched for the bars. 'No! NO Sensei! S'ool! S'ool!'

He hustled them all on as Donatello's cries reached peak rage and he began to scream in earnest. Above, the singing had ceased – he felt his heart go cold.

'Run,' he hissed to the others, and he ran, wrestling his hand over Donatello's mouth as he screamed and bit and kicked in his grip.

Splinter urged them onward and onward again, even as the little ones slipped and stumbled, until the tunnels were familiar and then more familiar still, and then there was their home. Home with a door, a thick, metal door he could close and lock behind them.

He leaned against it, heart racing, breath burning in his chest. No footsteps. No echoing voices. Just a quiet sobbing, the bundle in his arms expressing a grief too large for such a small soul.

'Hush. Hush now.'

'Sensei?' Leonardo's eyes went from his brother to Splinter's face. Raphael clung to Splinter's leg, Michaelangelo huddled behind him.

Splinter took a deep breath, tried to slow the frantic thrumming of his blood. 'It is all right, Leonardo.'

He went to where the mattress and neatly folded blankets made a seating area. It was shabby, but warm and dry. He sat, and laid Donatello down on the mattress. Aside from an indistinct 'NO,' and a muffled sniffle, the child fell quiet. With a hand on his back, Splinter felt him slip slowly into sleep.

He turned to the others, guilt twisting his insides. It was his loss of control that had frightened them. Splinter picked up Michaelangelo and settled him on his lap, where the child immediately knotted his hand in his fur.

'Everything is fine, my sons. I am sorry if you were frightened.'

'Why did we run?' Of all of them, Leonardo is the most verbal. Though he speaks slowly, he arranges his words with precision.

Splinter rests a hand on his head for a moment. 'Because I was startled. That is all.'

'I 'cared,' Michaelangelo piped up, and Splinter squeezed him gently.

'But you are not frightened any more, is it not so?'

'He's not,' Raphael said decisively – meaning of course that he was no longer frightened.

'Good. There is nothing to fear.' The lie was smooth and confident, and he stood and went to their makeshift kitchen – stacks of packing crates for cupboards, an old cardboard spool for a table – and reached for the jar of peanut butter. It has been the subject of intense, fevered interest since he introduced it a few weeks ago. 'Now, let us eat.'

Michaelangelo already grabbing for the lid, Splinter looked to Leonardo, looking anxiously at the sleeping Donatello. He put a hand on the little turtle's shoulder, steering him gently away. 'We will make him a sandwich, for when he wakes up.'

Some days, the children are a trial. This day, Raphael had refused to allow Michaelangelo to play with his favourite toy, a plastic horse missing one leg in a bright, slightly eye-searing red, and Leonardo was uncharacteristically recalcitrant, refusing to adhere to the most simple of requests. Such as 'Please do not climb on the table,' 'Please do not throw your food on the floor,' and 'Please do not hit Raphael with your food- No, or Michaelangelo- Leonardo, give me the cookie.'

This is why, when he looked to the corner where until recently Donatello had been leafing through the threadbare picture books he had scavenged for them and finds it empty, he cannot say with any accuracy how long it has been so.

Beyond, the door swung on its hinges, a mouth opening onto darkness.

He felt his heart go still.

'Raphael, watch your brothers.'

He walked to the door with a calm so complete it felt unreal. It was only when it swung shut behind him that he started to run.

His breath came short and sharp; his instinct telling him, at a level lower than the images running through his conscious mind – rushing water, cobbles slippery with grime, a small shape floating, motionless – where he will find Donatello.

As the tunnels sloped upwards and the light increased, notes rose like smoke on the damp air.

Rounding a corner, his legs weakened in relief.

The little turtle sat cross-legged under the grate, humming along, turning the pages of his picture book, fingers moving from colour to colour on the page, 'Red and yellow, oran' an' green, orange an' purple an' blue…'

'Donatello.'

Donatello looked up at him briefly, then back down to his book. '…an' purple an' blue…'

Splinter quashed his anger, and crossed to kneel in front of him. 'Donatello.' He stilled the page with a hand, and when the child would still not look at him, he tipped up the small face. Pensive brown eyes met his.

'Donatello, you must never do this again.'

The child tried to pull away, mumbling; Splinter held him in place.

'I must always know where you are. Always. Do you understand me?'

The little turtle nods.

'What do you say?'

'Hai, Sensei.'

He did not tell him he had done wrong – this one knew full well he had done so. When he picked him up Donatello did not struggle, but his head followed irresistibly the direction of the voices as they receded.

His brothers had been quiet when they returned. Raphael was seated, reading solemnly on the mattress, Michaelangelo and Leonardo on either side. Raphael looked up at him, a naked hunger for praise in his green eyes.

Tired to the bone, Splinter gave it nonetheless. How could he do otherwise? Donatello joined his brothers, and Raphael started the page afresh.

Splinter could have told the child any number of things. He could have promised some day. Perhaps with one of the others, he would have succumbed to temptation. But Donatello had always had an uncanny sense for lies.