  We present a holomorphic version of the bosonic string in the formalism of
quantum field theory developed by Costello and collaborators. In this paper we
focus on the case in which space-time is flat and construct a one-loop exact
quantization. Starting from first principles, we arrive at the critical
dimension as an obstruction to satisfying the quantum master equation.
Moreover, we show how the factorization algebra recovers the BRST cohomology of
the string and give another construction of the Gerstenhaber structure.
Finally, we show how the factorization homology along closed manifolds encodes
the determinant line bundle over the moduli space of Riemann surfaces.
