diff --git a/pkg/jia/handlers.go b/pkg/jia/handlers.go
index e786adb..3e61a87 100644
--- a/pkg/jia/handlers.go
+++ b/pkg/jia/handlers.go
@@ -1,6 +1,11 @@
 package jia
 
 import (
+	"fmt"
+	"log"
+	"regexp"
+	"strconv"
+
 	"github.com/slack-go/slack"
 	"github.com/slack-go/slack/slackevents"
 )
@@ -13,5 +18,56 @@ func HandleInnerEvent(slackClient *slack.Client, innerEvent *slackevents.EventsA
 	}
 }
 
+// TODO: Persist to Redis
+var (
+	lastValidNumber = 0
+	lastSender      = ""
+)
+
 func onMessage(slackClient *slack.Client, event *slackevents.MessageEvent) {
+	// Ignore messages that aren't in the target channel, or are non-user messages
+	if event.Channel != jiaConfig.ChannelID || event.User == "USLACKBOT" || event.User == "" {
+		return
+	}
+
+	// Attempt to extract a positive number from the beginning of a string
+	countPattern := regexp.MustCompile(`^\d+`)
+	matchedNumber, err := strconv.Atoi(countPattern.FindString(event.Text))
+	log.Println(matchedNumber)
+
+	// Ignore messages that don't have numbers.
+	if err != nil {
+		log.Println("Failed to retrieve number, skipping…")
+		return
+	}
+
+	// Reject if sender also sent the previous number.
+	if event.User == lastSender {
+		slackClient.AddReaction("bangbang", slack.ItemRef{
+			Channel:   event.Channel,
+			Timestamp: event.TimeStamp,
+		})
+		slackClient.PostEphemeral(event.Channel, event.User, slack.MsgOptionText(
+			"You counted consecutively! That’s not allowed.", false))
+		return
+	}
+
+	// Ignore numbers that aren't in order.
+	if matchedNumber != lastValidNumber+1 {
+		slackClient.AddReaction("bangbang", slack.ItemRef{
+			Channel:   event.Channel,
+			Timestamp: event.TimeStamp,
+		})
+		slackClient.PostEphemeral(event.Channel, event.User, slack.MsgOptionText(
+			fmt.Sprintf("You counted incorrectly! The next valid number is supposed to be *%d*.", lastValidNumber+1), false))
+		return
+	}
+
+	// Finally!
+	lastValidNumber = matchedNumber
+	lastSender = event.User
+	slackClient.AddReaction("+1", slack.ItemRef{
+		Channel:   event.Channel,
+		Timestamp: event.TimeStamp,
+	})
 }