Disclaimer: I don't own Jack, Donna, Martha, the Doctor, or Rose.
It was a peaceful moment on the TARDIS as the Doctor was lying underneath the TARDIS' console, tinkering away on the chameleon circuit for the fifteenth time that week, trying to remember how to fix it.
He flung his screwdriver down on the floor, frustrated. Suddenly a scream sounded throughout the console room, making the Doctor let out a strangled shout, shooting up from his position on the floor and hitting his head on the underside of the console. Cursing, he stood up and dusted himself off. He stroked the console apologetically and made his way down one of the corridors in search of the noise.
He paused and ran his fingers through his gravity-defying hair. As he was trying to think where the now non-existent scream had come from, he heard another, more masculine shout come from the library. He made his way inside and almost laughed at the sight.
Martha and Jack had somehow managed to get on top of one of the bookshelves and were huddled together, staring at a point on the floor with comically wide eyes. The Doctor smirked and followed their gaze to the floor. Upon seeing what they were looking at he was suddenly filled with fear and jumped up beside them with a rather girlish screech.
Rose came running in, only to stop short at the door and burst out laughing. The Doctor pointed a shaky finger to the creature on the ground. She looked where he was pointing and immediately screamed loud enough to bring Donna running in right as Rose fainted.
Surprisingly, Donna caught Rose upon entering and placed her down in a chair. She looked up at the Doctor, Jack, and Martha on the bookshelf and then at Rose passed out on the chair and then at the floor. She rolled her eyes and left the room, The Doctor, Martha, and Jack all shouting after her to come back and save them.
About ten minutes later Donna reappeared, carrying the sonic screwdriver that the Doctor had dropped in his mad dash to help Martha. She tossed it up to him, smirking, while he scanned the offending creature. Upon seeing the results he turned bright red in embarrassment and carefully climbed down the bookshelf. Donna was still standing there, laughing silently while the Doctor put his specs on, peering at the thing on the ground.
He glanced up at Donna, then at Martha and Jack on the bookshelf, and finally at Rose, unconscious in the chair, when he suddenly burst out laughing. He looked at Donna and she dissolved into giggles, drawing Jack and Martha down off the top of the bookshelf to see what it really was. Jack turned bright red and Martha started giggling.
The Doctor, calmed down and, picking up the offending item off the floor, said, "It's a piece of lint." Then he broke down into laughter, the others following suit quickly.
They left the library together, laughing and talking about their sheer stupidity, leaving an unconscious Rose still in the chair.
Let it be said they all were at Rose's mercy for little less than a week for leaving her there alone with the supposed "killer fluff."