  We present the results of our comprehensive investigation on the
antiferromagnetic heavy-fermion superconductor Ce$_3$PtIn$_{11}$ carried out by
means of electrical transport, heat capacity and ac magnetic susceptibility
measurements, performed on single-crystalline specimens down to 50 mK in
external magnetic fields up to 9~T. Our experimental results elucidate a
complex magnetic field -- temperature phase diagram which contains both first-
and second-order field-induced magnetic transitions and highlights the
emergence of field stabilized phases. Remarkably, a prominent metamagnetic
transition was found to occur at low temperatures and strong magnetic fields.
In turn, the results obtained in the superconducting phase of Ce$_3$PtIn$_{11}$
corroborate an unconventional nature of Cooper pairs formed by heavy
quasiparticles. The compound is an almost unique example of a heavy fermion
system in which superconductivity may coexist microscopically with magnetically
ordered state.
