Lucious was a part of the plan,because he had to put the Diary into Ginny's cauldron so he obviously knew what was going to happen, and Dobby probably picked it up, being around the house and all.

didn't lucius plan slipping the diary into some hogwart's student's bag so he could help Voldemort come back as his sixteen year old self?

I totally agree with you! Plus Lucius hated the Weaslys So it would fit...

good point.
i mean DUH Dobby overheard the Malfoys.
but i thought Lucius didnt know Voldemort was back. he didnt believe it.
so he had not contact with the dark lord..so how did he kno something was going to happen?

read the thread about Why did Riddle give Ginny the diary.
lots of people, including me, said Lucius gave Ginny the diary because he needed things that could connect him to the dark lord out of his house and he hated the Weasleys so he just gave it to them.
he might have known it held something..but he didnt kno what.

so how would they kno something was gonna happen?