V star Miranda Hart will be attempting a series of "madcap challenges" for Comic Relief.
The funnywoman, 40, will race against the clock in a different UK city each day for five days to complete the mystery tasks.
Miranda will only discover what she has to do hours before the cash-raising stunts, every morning on Chris Evans's Radio 2 Breakfast Show.
Viewers of BBC1 programme The One Show will see how the Miranda star fares live at the end of each day.
A Comic Relief spokesman said: "Whether it involves romance, sequins or animals - which it might not - she will definitely be calling upon members of the public to help her on her way."
Miranda said: "This year for Red Nose Day, I will be doing... something! I have absolutely no idea what, I just hope that the good people at Comic Relief aren't too hard on me and that the British public get behind me and help me raise heaps of cash for Red Nose Day!"
The Call The Midwife actress will visit Newcastle on March 11, followed by Manchester, Birmingham, Oxford and then London on Red Nose Day, which is Friday March 15, as part of The BT Red Nose Challenge: Miranda's Mad March.
BT will be donating 1p to Red Nose Day for every call made from a BT home phone line or BT business line on March 14.