He comes when Luke is asleep and the world is still.
The knock at the door. The one word, his way of saying her name.
"Sarah."
She doesn't say a word but leads him inside to the couch, where she curls into his arms. They sit there for hours, unmoving, basking in the comfort of the other. No words need to be said. Each know of loss. It lessens being shared.
He first came the night after the aborted wedding. Sitting on the couch in her grief, she heard the familiar whine of the Tardis. Slowly, she walked to the front door, opened it, and watched him walk towards her.
"I remembered what Davros said," he said to her when he's close enough to talk in a soft voice. "About how I always run away. I can't run anymore, Sarah. I just can't."
She invited him in and made them tea. They settled on opposite sides of the couch. He asked her to tell him about Peter, so she did. She then asked him what happened to everyone after he dropped her off and he told her about saying goodbye to Rose again. He spoke in an even voice but knowing him so well, she could see how deeply it hurt him. Then -
"You said no one will ever forget me. But Donna has to. It will kill her if she remembers."
On impulse, she reached over and grabbed both his hands. He looked down at their intertwined grasp and he broke. She closed the distance between them and held him as through broken words he described what had happened to Donna. It became too much for both of them and they spent the night crying in each other's arms.
As the dawn broke, they separated, an unspoken mutual agreement .
"Luke will be up soon. I don't want him seeing me - us - like this."
"Then I must be on my way." He walked to the door and she followed to open it. As he walked out, she says,
"My door is always open. You know that, don't you?"
"Oh, my Sarah Jane." He wrapped her in one last hug, then turned and left. She watched the Tardis disappear, not knowing if he would be back. Hoping he would be.
Luke knows, of course, although he acts like he doesn't. He makes a point of pretending to be asleep as soon as it would make sense for a teenager to be. But he watches the Doctor leave every night, seeing his steps that little bit lighter.
Just like his mom's.