Tonks was sick of following false leads on Sirius when she could be doing something useful for the Order but she worked for the Ministry and they were stubbornly refusing to believe Voldemort had returned. The Auror office was spread thin with the new prison break and so she'd been taking many of these calls alone.
She was a skilled witch and highly trained Auror. Then why was it she felt so uncomfortable when she reached the address where Sirius was reported to have been seen? It was an abandoned factory on a dank and deserted street like many of the false sightings. Then why does this one not feel right? She wished Sirius really was there.
The unnecessary fear made her angry. Trying the door she found the lock broken. Entering cautiously Tonks used the light from the open door to see. It was a large room with a tall ceiling. Just how large she couldn't tell because the far end was completely in shadow as were the walls and corners. The small, narrow windows set high along the walls were so covered in filth no light could shine through. The room appeared to be empty apart from many large black shapes along the walls; she took for cabinets of some sort.
The door swung shut behind her with a heavy thud. When she heard it she turned but found it was in complete darkness, as was everything else. What was more she could hear footsteps along the walls. It would have been easy enough to light her wand but she was unsure how many people were lurking in the shadows and didn't want to be an easy target. If she couldn't see them chances were good they were having the same trouble seeing her.
Another pair of footsteps joined the first. They seemed to be circling her. Stepping back where she knew the door to be she tried every spell she thought might open it but it wouldn't budge. She found something setting along the wall, a bottle, and threw it as far as she could.
When it crashed Tonks could hear two sets of footsteps follow it and she moved with them using their sound to mask her own footfalls.
"Where is she?"She heard a female voice hiss, then a small cry of pain from an unquestionably male voice.
"Damn it." Clearly he had found the broken bottle. "Enough games. Let's catch her and be done with it."
The female voice replied almost childlike. "I like playing with her." She laughed an insane cackle.
Bellatrix. Tonks realized and the male voice sounded like Lucius Malfoy and they seemed to be the only others in the room. The voices were still at the far end where the bottle had landed. Creeping along a wall she hid behind one of the large black shapes she had seen earlier. Pointing her wand at the window just over where her foes should be, she whispered "Expulso."
The glass burst and light spilled in, illuminating them. They were ready, their wands out but Tonks was still hidden in shadow. "Stupefy." she whispered but Bellatrix had moved closer and was once again hidden from view. Breaking another window she saw her again.
Lucius seemed to have noticed what direction her spells came from and fired some of his own from out of the darkness. The cabinet effectively shielded her at first but soon it caught fire. Bellatrix saw her then.
"Sectumsempra!" she missed but cackled anyway.
"No." Lucius shouted at her. "Do not spill her blood. It is too precious." For the first time Tonks noticed every spell he had flung at her was meant to incapacitate. To stop her without doing her any real harm. "My blood?" she asked more to stall than anything. She had a fair idea of their plan, barmy as it was.
"Yes my dear niece, your blood." Bellatrix seemed eager to talk.
"That's enough." Lucius bellowed, but she ignored him.
"Your metamorphmagus blood is quite useful to us. Imagine the Dark Lord's army appearing anyway they like. They could be anyone, anywhere and no one would be the wiser."
Tonks could have laughed. "That's just a myth. My blood is no good to you. No one has ever made that potion work."
She'd heard of this before, a potion made from the blood of a metamorphmagus that with only a sip their abilities would pass on to the drinker. It would be ideal to conceal a large group of people but no one had ever succeeded in producing it. For one thing the donor had to die for it to work. It was said the last drop was crucial.
"Ahh, but we've found a way." Bellatix whispered as if sharing a secret. "Enough", Lucius was losing patience, "if she should…"
At that moment the door slammed open and Snape strode in. When the two Death Eaters looked at him Tonks used their moment of distraction to stun Bellatrix. Lucius had his wand trained on Tonks but Snape sent a hex that just barely missed. Lucius, looking thoroughly outraged, grabbed an unconscious Bellatrix and Dissapparated them both.
"Are you alright?" Snape asked, putting his hand on her shoulder to steady her as she seemed a bit shaky.
"I'm fine. But you wouldn't believe what they're planning."
"Are you sure they were telling the truth?" asked Moody after Tonks recalled what happened only an hour before at the factory to the assembled members of the Order.
"I don't know. Lucius sounded really upset she was talking but I suppose it could have been lies, just meant to throw us off track." Tonks explained.
"I do not believe they were lies." Snape interrupted. "Bellatix believed they had her but I can find out for certain."
"Why did you save her?" Lucius snarled. "You ruined our plan."
Bellatrix nodded vehemently. Snape was equally as furious. After all, they had attacked the woman he loved. Loved? Did he love her? "Why didn't you inform me of this plan, then perhaps I wouldn't have thought to save her?" Snape's voice had dropped dangerously low.
Lucius seemed to notice this and choose to speak a little softer. "That still doesn't answer why you chose to help her in the first place."
Snape's voice rang out, echoing throughout the cavernous room. "I chose to help her, Lucius because Dumbledore paired us to work as a team and if I had not intervened it would have tipped him off to the fact that I am a spy."
He waited as Lucius took in his explanation unsure himself whether it all made sense. It was the best he could do. He hoped that if he projected enough confidence and yelled enough that Lucius would not analyze his words too deeply. Looking at him he could tell Lucius seemed to believe him. Bellatrix was mumbling to herself, but it was unclear if she was angry or merely conversing with a voice only she could hear.
Turning the questions around once more he asked, "Why wasn't I informed?"
Lucius looked almost frightened. "You are in good with the Dark Lord; we needed the credit after that incident at the nursery."
Snape feigned indignation. "Credit?" he snarled. "This should not be about credit. This may benefit the Dark Lord. That is all that matters!"
"It doesn't matter anymore." Lucius sounded defeated. "Now that she…" he gestured widely at Bellatrix "told our target of the plan."
"On the contrary, Instead of simply killing the witch so the Dark Lord cannot have her, Dumbledore, the old fool, will be concerned for her welfare. We may use this to our advantage."