Her frank answer completely assured me that she was speaking the truth.

At the same time she explained that she had been late with her theme on account of mislaying it.

She had written it two days before and placed it in her desk.

Then it had mysteriously vanished and suddenly reappeared in the same pigeonhole in her desk in which she had placed it.

She assured me that directly she found it she took it to the box.
Your theme is so suspiciously similar to hers that it is hardly possible to believe it to be merely a coincidence.