During their little eavesdropping session, Harry and Ron heard Professor Lockhart say that he was going to try to get into the Chamber of Secrets to defeat the monster and save Ginny. So they run to his office to give him their intel.
Turns out that not exactly what Lockhart is up to. He's packing to leave. Lockhart finally admits that he isn't the brave and daring wizard he's made himself out to be. He's a total fraud.
He does tell Harry and Ron that he's pretty good at erasing memories though and that's exactly what he's going to do to them. But Harry and Ron are able to overpower him. Wow. Lockhart really is a rubbish wizard.
So Harry and Ron take Lockhart prisoner and lead him down to the girls' bathroom. There, they ask Moaning Myrtle how she died.
Myrtle explains that she was in a bathroom stall crying when she heard a boy outside speaking a strange language. When she came out all she saw was a pair of yellow eyes over by one of the sinks and then she was dead. Poof. Just like that.
Harry looks at the sink and sees a little snake on it. He speaks in Parseltongue and the sinks pull back to reveal a hole in the floor.
(It's crazy, but this means the bathrooms at Hogwarts haven't been remodeled in 1,000 years. Seriously, Dumbledore, get on this renovation.)
The boys push Lockhart down into the hole and then jump in after him.