A unique solvability of the inverse initial problem for a time-degenerate fractional partial differential equation is proved. Using the method of variable separation, we obtain the Cauchy problem for the fractional differential equation involving the bi-ordinal Hilfer derivative in the time variable. The authors present the solution to this Cauchy problem in an explicit form via the Kilbas–Saigo function. Further, using the upper and lower bounds of this function, the authors prove the uniform convergence of the infinite series corresponding to the solution of the formulated inverse initial problem. Keywords: inverse-initial problem, degenerate PDE, bi-ordinal Hilfer operator, Kilbas–Saigo function.