  We propose a worldsheet description for the ${\rm AdS}_5\times {\rm S}^5$
string theory dual to large $N$, free ${\cal N}=4$ super Yang-Mills in four
dimensions. The worldsheet theory is a natural generalisation of the recently
investigated tensionless string on ${\rm AdS}_3\times {\rm S}^3\times
\mathbb{T}^4$. As in the case of ${\rm AdS}_3$ it has a free field description,
with spectrally flowed sectors, and is closely related to an (ambi-)twistor
string theory. Here, however, we view it as a critical $N=4$ (closed) string
background. We argue that the corresponding worldsheet gauge constraints reduce
the degrees of freedom to a finite number of oscillators (string bits) in each
spectrally flowed sector. Imposing a set of residual gauge constraints on this
reduced oscillator Fock space then determines the physical spectrum of the
string theory. Quite remarkably, we find compelling evidence that this
prescription reproduces precisely the entire planar spectrum - of single trace
operators - of the free super Yang-Mills theory.
