Leonard agrees to sign for a package for Penny, but when it turns out to be a massive unassembled entertainment unit, he and Sheldon must muscle the box up several flights of stairs - a task for which they are ill-equipped.
Once in Penny’s apartment, Sheldon is appalled at Penny’s housekeeping practices and takes it upon himself to clean up somewhat until Penny gets home. That night, the fuss budget in him can’t stand it anymore, so he sneaks back to finish the job (while Penny is asleep). Against his better judgement, Leonard helps.
When Penny wakes to find that the guys were in her place, she feels violated and is furious, setting back Leonard’s plans for romantic bliss.