The Rulebook Says wrote:
Playing Mice and Mystics as a Campaign
Playing as a campaign allows players to preserve the progress of their mice from one chapter to the next. The following guidelines are suggested for playing mice and mystics as a campaign:
1. Mice keep their story achievements and abilities earned from previous chapters.
2. Mice can keep only 1 search card that they had at the end of the previous chapters. This 1 card is in addition to their starting equipment cards. Mice will always begin a new chapter with their starting equipment cards.
3. Mice do not keep any party items, individual achievements, or cheese from previous chapters.
So you can keep the elixir or the knife, but not both. Unless you had another mouse who didn't have anything, I suppose; giving an item to another mouse is a free action. This also applies to Trick cards.