  One of the aims of the BCool programme is to search for cycles in other stars
and to understand how similar they are to the Sun. In this paper we aim to
monitor the evolution of $\tau$ Boo's large-scale magnetic field using
high-cadence observations covering its chromospheric activity maximum. For the
first time, we detect a polarity switch that is in phase with $\tau$ Boo's 120
day chromospheric activity maximum and its inferred X-ray activity cycle
maximum. This means that $\tau$ Boo has a very fast magnetic cycle of only 240
days. At activity maximum $\tau$ Boo's large-scale field geometry is very
similar to the Sun at activity maximum: it is complex and there is a weak
dipolar component. In contrast, we also see the emergence of a strong toroidal
component which has not been observed on the Sun, and a potentially overlapping
butterfly pattern where the next cycle begins before the previous one has
finished.
