We have examined Cluster spacecraft 1 (C1) magnetic field and particle observations from 27 August 2001 when the Cluster spacecraft separation was less than 0.27 RE and the spacecraft were located near the equatorial plane of the postmidnight magnetotail at xGSM ~ -19 RE and yGSM ~ -2.1 RE. A detailed examination of observations recorded between 0350 UT and 0435 UT is performed with special emphasis on the plasma flow reversal from strong tailward flows to earthward flows in the approximate time interval 0400 UT to 0410 UT. Evidence for the expansion phase onset of an isolated substorm between ~0406 UT and ~0408 UT was reported by Baker et al. <2002>. During the 0400--0410 UT interval, C1 was located in the plasma sheet where the observed plasma density N ~ 0.3 cm-3 and the plasma ion ¿ ~ 1.5. The Wal¿n analysis applied to the tailward flow interval from 0400:36 UT to 0403:14 UT is consistent with the ions being accelerated to ~73% of the Alfv¿n speed across a slow-mode shock connected to a near-Earth neutral line located on the earthward side of the spacecraft. The occurrence of a small plasmoid-type magnetic flux rope during the leading edge of the tailward flows provides further support in favor of an active region of magnetic reconnection earthward of Cluster. The more field-aligned earthward flows between 0406 UT and 0408 UT, however, failed to satisfy the Wal¿n test. Rankine-Hugoniot analyses of upstream and downstream plasma and magnetic field parameters confirm the presence of a slow-mode shock in connection with the passage of the tailward flow region but not with the 0406 UT to 0408 UT earthward flow interval. The confirmed shock satisfies the critical slow-mode requirements: MI* ≤ 1.0 and MSM* > 1.0 on the upstream side and MSM* -19 RE of the near-Earth magnetotail.