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Laminar-turbulent transition mechanisms induced by a harmonic point source disturbance in a flat-plate boundary layer with adverse pressure gradient are investigated by fourth-order accurate spatial direct numerical simulation based on the complete three-dimensional Navier--Stokes equations for incompressible flow. The disturbance is introduced into the two-dimensional base flow by time-periodic simultaneous blowing and suction within a circular spot at the wall to quietly mimic the momentum input by an active loudspeaker below a hole in the surface in respective experiments. Thus a wave train consisting of pure Tollmien--Schlichting waves of a single frequency and a large number of obliqueness angles is stimulated, and its downstream evolution in both physical and spectral space is investigated. A breakdown scenario dominated entirely by oblique modes is observed that shows a span-wise peak/valley amplitude splitting with the valley plane at the centerline of the wave train. Dominant vorticity structures develop off centerline, and a clear-cut M-shaped structure is formed in final stages related to the wall shear being a footprint of a first pair of [lambda]-vortices in the flow.