  In this short note, we give a self-contained derivation of the formula for
the $2$-point full-plane Ising spin correlation function under massive scaling
limit in terms of a third Painlev\'e transcendant. This formula, first derived
in a celebrated work of Wu, McCoy, Tracy, and Barouch, was subsequently
reformulated in terms of the theory of isomonodromic deformation by Sato, Miwa,
and Jimbo. In view of recent developments in the discrete analysis which have
enabled, in particular, a convergence proof of spin correlation functions on
isoradial lattice, we give a concise and rigorous account of the continuous
theory in the same framework yielding this iconic result.
